Microstructural evolution of liquid metal embrittlement in 1.5-GPa-grade Zn-coated hot-press-forming steels

Zn-assisted liquid metal embrittlement (LME) in hot press forming (HPF) steels is attributed mainly to the cracking inside both Zn coating and HPF substrate, but very few studies on how the Zn coating composed of Fe–Zn intermetallic compound of Γ-Fe3Zn10 and Zn-containing ferrite (α-Fe(Zn)) and alloying elements influence the LME cracking have been conducted. In this study, mechanisms of LME were investigated by using laboratory-scale HPF simulation tests in relation with microstructural evolutions of Zn coating and LME cracking. The coating and steel substrate were composed mostly of α-Fe(Zn) and full martensite, respectively, in both reference (Ref) and 1-wt.%-Si-containing (1Si) HPF steels, while the Γ phase was additionally formed at the 1Si steel. The high-temperature tensile test results indicated that the loss of elongation increased with decreasing exposure time near or at 900 °C and with increasing test temperature or Si content. Based on the equilibrium Fe–Zn binary phase diagram, the amount of liquid remained after the austenitization was larger in the 1Si steel than in the Ref steel because the addition of 1 wt% Si retarded the Fe–Zn alloying reaction between the Zn coating and steel substrate to raise the susceptibility to LME. Thus, the Si content should be carefully controlled by appropriately using the HPF process parameters such as the time of austenitization at 900 °C to sufficiently achieve the Fe–Zn alloying reaction.